The whole line in a .meta file is a serialized PHP array. See
http://php.net/unserialize
The number itself is a unix timestamp. The number of seconds elapsed since the "Epoch" (1970-01-01 00:00 GMT).
To convert to a date, see,for example,
https://stackoverflow.com/questions/10040291/converting-a-unix-timestamp-to-formatted-date-string .